Add ability to A/B test features

[patcon]

Reading through #2052 and #2053, thinking that it would be great to have real data on all the little decisions being made on UI and copy. Would be great to understand how callouts and little changes might affect conversion on the real things we care about :)

This seems to be the only platform-agnostic library out there:
https://pypi.python.org/pypi/swab

keywords: multivariate, split

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[rohitpaulk]

We've got optimizely set up now. #2857

[webmaven]

I consider Optimizely a temporary (that is, for several months) solution, as costs rise dramatically for multi-page and multi-variate tests (which we will want to do eventually) and with volume. Besides the above mentioned Swab, we should also consider Planout: http://facebook.github.io/planout/

[chadwhitacre]

Closing in light of our decision to shut down Gratipay.

Thank you all for a great run, and I'm sorry it didn't work out! 😞 💃